Balanced Rotating Accembly - Throwing Weight

Keep Everything In Balance

Today's race-bred engine is a highly advanced piece of engineering. A couple of grams here and there in the reciprocating department may not seem like such a big deal, but in reality, it can make or break a potent powerplant. Case in point:

A typical Sunday cruiser with a mostly stock powerplant won't care; however, in the world of big horsepower and upper-rpm-slinging street/strip cars, it makes all the difference in the world. These cars are making serious power, and keeping that mill together is all contingent on what's going on inside the block

A balance job isn't to be taken lightly. We are pushing the limits of physics, design, and rpm. Blend all those together and it leaves little room for error. Tolerances are being measured in thousandths of an inch, and pistons are weighed in hundredths of a gram. Any miscalculation here can lead to torched bearings, a sloppy-vibrating mess, or worse.

"There are certain balances for certain applications," says Mike Consolo of QMP in Chatsworth, California. He reiterated that its important to know what engine combination is being used in order to decide what type of balance to throw at it. Since this isn't something that can be done in the comfort of your own garage, it's important to find a machine shop that has the equipment and know-how to get the job done.

For this balance job, we delivered everything, including the crankshaft, pistons, rods, rings, and wristpins. Once there, Consolo took the reigns and steered this balance job into something more drag-race-worthy.

Bottom LineKeep your bottom end buttoned up and spinning without a hiccup.

Cost (Approx)Starting at $150

Weighing InHow are the bob weights determined? It's simple actually. Consolo added all of the components together. He made sure to add the large end of the rods twice and the bearings twice since two rods swing together on each rod journal.

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Balanced Rotating Accembly - Throwing Weight

To get things started, Consolo unwrapped all the rotating components and began to weigh them. He weighed everything, including the pistons, piston rings, wristpins, ring-locks, piston rods, and rod bearings. He also took into account the weight of oil as it gets dispersed throughout the rotating assembly.

Next, Consolo matched the weights he had measured and added the bob weights to each of the crank's rod journals. The bob weights will simulate the rotating weight of the engines components on the crankshaft while it is spinning on the balance machine.

To obtain an accurate measurement on the balance machine, an angle finder is used to determine a starting point. Consolo placed the crank into the machine. He then spun the crankshaft until the front rod journal (No. 1) was sitting vertically-almost like obtaining TDC. Consolo attached the first bob weight and used the angle finder to determine zero degrees. From there he rotated the crankshaft 180 degrees, attached the next bob weight to the last rod journal (No. 4), and set that bob weight with the angle finder to zero degrees. Next, he spun the crankshaft another 90 degrees, set the bob weight onto the next rod journal (No. 3), and set that one to zero degrees as well. Finally, Consolo rotated the crankshaft another 180 degrees and set that bob weight onto the second rod journal (No. 2).

Once all of the individual weights had been accounted for and the bob weights added, Consolo spun the crankshaft on the balance machine. The machine spins the crank at about 450-500 rpm for 15-30 seconds depending on how out of balance the crankshaft is. When the spinning is over, the computer records and reports how much weight to remove or add to the crankshaft and where. In our case, we didn't have to remove any weight whatsoever.

Starting at the rear of the crankshaft, Consolo centered-punched it to begin drilling. He first drilled a pilot hole and eventually worked his way up to using a 1-inch drill bit. Consolo ended up drilling a 1-inch hole 1 inch deep. He spun the crank again on the machine, and it was determined that the rear was still out of balance. More weight had to be removed, so he ended up drilling two additional 1-inch holes 1 inch deep.

The Sunnen balance machine is a beautiful piece of machinery. It makes balancing relatively straightforward, telling the machinist where to drill, what size drill bit to use, and how much weight to remove. 1. How much weight to remove from the front of the crankshaft, in grams.2. What angle the crankshaft needs to be at to remove weight from the front.3. What angle the crankshaft is currently sitting.4. What angle the crankshaft needs to be at to remove weight from the back.5. How much weight to remove from the back of the crankshaft.

Through additional spinning, it was learned that two more holes needed to be drilled to complete the balance. If weight had to be added, the balance job could be much more expensive because Mallory slugs, which are dense pieces of metal, would need to be used to add weight to a balance job. Be warned: The slugs run about $100 a piece.

Every little bit helps. Creating a smooth finish on the crank's rod journals should also be a part of the balance job. Here, Consolo placed our big-block crank into the lathe and began to spin the crank. Using a specially designed crank polisher and 1,000-grit sandpaper, he polished the rod journals, creating a burr-free environment for the bearings that will allow oil to distribute much more freely.

At QMP, no crank leaves the shop without its drill holes chamfered and properly cleaned. Using an air-powered sanding disc, the edges were sanded smooth to rid the crank of any flash. Then a chamfering tool beveled the rim of each hole. An adequate wash in solvent while paying special attention to cleaning each oil galley with a brush will leave this crank ready to be dropped into its new home.